Aluminum based anodes and process for preparing the same

ABSTRACT

Disclosed is a method for preparing an aluminum-based anode, including at least one alloying element, prepared using solid solution heat treatment, in addition to plastic deformation, artificial aging, or a combination thereof.

BACKGROUND OF THE INVENTION

Aluminum is known to have a relatively high electrochemical capacity,and therefore, is highly attractive for use as an anode in batteries,including aluminum-air batteries, in which the aluminum reacts withoxygen from the air. However, the use of such anodes is limited due tothe corrosion of the anode, which occurs mainly at open circuit voltageand at low current density by reaction of the aluminum anode (Al-anode)with the electrolyte. Such corrosion causes the consumption of theAl-anode, without the generation of electrical power, thus causing thedeterioration of the battery and highly limiting the shelf life thereof.

Several attempts have been made to suppress the corrosion of theAl-anodes, including changing the metallurgical properties of theAl-anode and adding corrosion inhibitors to the electrolyte. One suchattempt is that of changing the metallurgic properties of the anode byalloying the Al with other elements. However, these attempts were notvery successful.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those of ordinary skill in the artthat the invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components, modules,units and/or circuits have not been described in detail so as not toobscure the invention.

Embodiments of the invention are directed to a method for preparing analuminum (Al) based anode. The anode according to this embodiment isprepared by smelting an alloy from aluminum and at least one alloyingelement, such as magnesium, so as to provide a supersaturated solidsolution metastable phase, producing strips of the smelted alloy,treating the strips by solid solution heat treatment and decomposing themetastable supersaturated solid solution phase by artificial aging,plastic deformation, or both, providing a Al based anode.

According to some embodiments, an Al—Mg anode is prepared. According tothese embodiments, the amount of the magnesium in the Al—Mg alloy doesnot exceed its maximum solubility concentration in the supersaturatedsolid solution metastable phase, and therefore, the amount of themagnesium in the alloy is 0.5 to 10% w/w. In some embodiments, thepercentage of Mg may be 2-4% w/w.

In some embodiments, the means used for producing strips of the smeltedalloy may include hot rolling, cold rolling, stamping, pressing andmachining. In some embodiments, the solid solution heat treatment mayinclude heating the strips to a temperature of 400-500° C.; maintainingthis temperature for 1-5 hours; and quenching into a liquid media. Insome embodiments the decomposition of the metastable supersaturatedsolid solution phase may be performed by rolling.

Further embodiments of the invention are directed to a an Al based anodeprepared by smelting an alloy from aluminum and at least one alloyingelement so as to provide a supersaturated solid solution metastablephase, producing strips of the smelted alloy, treating the strips bysolid solution heat treatment and decomposing the metastablesupersaturated solid solution phase by artificial aging, plasticdeformation, or both.

Further embodiments are directed to Al based anodes, wherein thealloying element is Mg, having a coulombic efficiency of at least 85% ata temperature of 40-50° C. Further embodiments are directed to Al basedanodes, wherein the alloying element is Mg, having a coulombicefficiency of 87-91% at a temperature of 40-50° C.

Various aspects of the invention are described in greater detail in thefollowing Examples, which represent embodiments of this invention, andare by no means to be interpreted as limiting the scope of thisinvention.

EXAMPLES

The results presented below are based in the statistical analysis of theresults of several experiments.

Example 1 Corrosion Rate after SSHT Before Decomposition

1.5 kg of Al—Mg 2.5% alloy was smelted from 1.462 kg of aluminum (purity99.99%) and 0.038 kg of magnesium (purity 99.999%) in a graphitecrucible in an induction furnace under a protective atmosphere.Magnesium and other alloying elements were wrapped up in Aluminum foiland plunged into already melted Al. The melt was poured out into a steelmould of 150×15×260 size. Before casting the melt was vigorously stirredby graphite rod. The same procedure (besides the alloy's composition)was used for smelting of all the mentioned below Al base alloys.

Casting stress relief annealing was carried out at 350° C. for twohours, cooled down to room temperature and then the strips were rolledin a duo rolling mill to a thickness of 3.5 mm. This annealing procedureis optional and may be performed to reduce internal stress and tohomogenize the structure. SSHT of the strips was carried out in anelectric batch type furnace with circulating air. The strips were heatedup to 415° C., maintained at this temperature for four hours andquenched in water to room temperature. A rolling duo mill having a rolldiameter of 300 mm was used for rolling the ingots with different ratesof deformation.

The test samples had a size of 30 mm diameter and 2.5 mm of thickness.Aluminum samples were machined directly from the ingots while alloysamples were machined from the strips and later subjected to solidsolution heat treatment, and optionally an artificial aging process. Theartificial aging process was carried out in a batch furnace at 150-200°C., depending on alloy composition, under an air atmosphere (thespecific temperatures used during the artificial aging process for eachalloy are presented in Table III below).

The corrosion value, coulombic efficiency and polarization tests werecarried out in electrochemical half-cells in 4M KOH at 50° C. Thecorrosion value at OCV and coulombic efficiency in galvanostaticexperiments were measured by weight loss. Here and further all thepotentials were measured vs. Hg/HgO reference electrode with IR dropcorrection. Before each test the sample's working surface was polishedby the SiC abrasive paper grit 600, followed by a fine aluminasuspension AP-A polishing.

The corrosion rate at OCV for Al and Al based alloys after solidsolution heat treatment is as follows:

TABLE I Corrosion rate Alloy composition SSHT (° C.) (mg/cm² · min) Al99.99 — 0.81 Al 99.9 — 0.92 Al—Mg 2.5% 415 0.52 Al—Mg 3.8% 415 0.51Al—Mg 6% 415 0.63 Al—Mg 2.7%—Si 0.7% 415 0.83 Al—Mg 2.1%—Ge 0.6%—Ga 4150.85 0.3% Al—Si 1.2% 560 0.82

As shown in Table I, performing solid solution heat treatment for Al—Mgalloys having an Mg content of less than 4% results in a significantdecrease in the corrosion rate in comparison to pure Al, as well asother Al based alloys. It was further found that the corrosion productsof Al—Mg alloys having up to 4% Mg completely dissolve in an alkalinesolution, and therefore, the working (corroded) surface of these alloysis smooth and clean. In contrast, it was found that the other alloys,including the Al—Mg alloy with 6% Mg, form a porous layer of corrodedproduct on the working surface of anode. This porous layer can notablyincrease the anodic polarization, as will be shown below. Additionally,the corroded products may migrate into the electrolyte to form a veryfine suspension, further disrupting the efficiency of the anode.

Example 2 Corrosion Rate after Artificial Aging

The alloys prepared according to the procedure detailed above wereadditionally subjected to artificial aging. The corrosion rate .vs. thetime of aging of the various alloys is shown below in Table II.

TABLE II Aging Time of aging/Corrosion Alloy composition (° C.) rate(h/(mg/cm²min)) Al—Mg 2.5% 150 90/0.47; 175/0.4; 220/0.42 Al—Mg 3.0% 150170/0.38; Al—Mg 4.5%—Ga 0.5% 150 90/0.58; 190/0.52 Al—Mg 6% 150 90/0.60;175/0.49 Al—Mg 2.7%—Si 0.7% 160 90/0.77; 175/0.63 Al—Mg 2.1—Ge 0.6—Ga160 90/0.71; 175/0.59 0.3 Al—Si 1.2% 190 90/0.79; 175/0.65

Example 3 Comparison Between Corrosion Rates when Using Artificial Agingand Plastic Deformation

1.5 kg of an Al—Mg 3.4% alloy was prepared according to the proceduredescribed in Example 1. The ingot of size 150×15×260 mm was rolled tothe strips having thickness 4.5 mm. The solid solution heat treatmentfor these strips was carried out as follows: heating up to 415° C.,maintaining at this temperature for 4 hours and quenching in water atroom temperature. After quenching the strips were rolled from thethickness of 4 mm to 1.1-1.2 mm and then some samples (Group A) wereelectrochemically tested and some of them (Group B) were subjected toaging process at 150° C., before electrochemical testing.

The results show that the average corrosion rate for Group A samples was0.33 mg/cm²·min The results of the corrosion test for samples of Group Bare summarized in Table III

TABLE III Time of aging, h 0 67.5 133.5 200 263 Corrosion rate,mg/cm2/min 0.34 0.34 0.33 0.32 0.34

From comparing the results presented in Examples 2 and 3 (Group A), itcan be concluded that solid solution heat treatment+plastic deformationby rolling of the Al—Mg alloy, having supersaturated solid solutionstructure, results in notably lower corrosion rate as compare to thesolid solution heat treatment+aging (see Table II). It should be alsoemphasized that a plastic deformation by rolling is much less time andlabor consuming compared to the low temperature, long time agingprocess. Further, the rolling also provides the flattening of thestrips, which are deformed after the solid solution heat treatment. Bycomparing the results of Group A (including plastic deformation with noartificial aging) and Group B (including both plastic deformation andartificial aging) it is concluded that once plastic deformation isperformed, the additional artificial aging process does not change thecorrosion rate.

Example 4 Polarization Data of Several Alloys

Polarization data for the Al—Mg alloys with Mg content 2.5-4.0% aftersolid solution heat treatment+aging or solid solution heattreatment+deformation do not differ markedly. However, when comparing apure aluminum anode with an Al—Mg 2.5% anode and an Al—Si 1.2% anode(both prepared according to the procedure described in Example 2), it isshown (see Table IV) that the current density of the Al—Mg 2.5% anode ishighly improved.

TABLE IV Current density, mA/cm2 0 25 50 100 150 200 Al (99.99)/ −1670−1605 −1525 −1370 −1130 −905 Potential, mV Al—Mg 2.5%/ −1760 −1702 −1630−1518 −1380 −1135 Potential, mV Al—Si 1.2%/ −1476 −1420 −1374 −1252−1084 −940 Potential, mV

As shown in Table IV, the Al—Mg alloy provides markedly more negativepotentials as compared to the pure Al, i.e., the presence of Mg in thecrystalline structure of the Al—Mg alloys provides more negative anodepotentials. The polarization of the Al—Si alloy is much higher than boththe pure Al and the Al—Mg anode, which may be caused by the formation ofa corroded product on the working surface of the anode.

The anode coulombic efficiency, % for pure Al (99.99), for an Al—Mg 3.4%alloy prepared using solid solution heat treatment+75% deformation andfor an Al—Mg2.7%—Cr0.19%—Mn0.04% prepared without any solid solutionheat treatment or deformation is as presented in Table V:

TABLE V Current density, mA/cm² 6 12 25 50 100 Coulombic efficiency ofAl (99.99), % 4.9 15.2 40.4 62 82 Coulombic efficiency of 6.8 19 45.3 6888 Al—Mg 3.4% alloy Coulombic efficiency of Al—Mg 17.4 31.4 2.7%—Cr0.19%—Mn 0.04% alloy

As shown in Table V, at small current densities (or at more negativepotentials) the coulombic efficiency of heat treated alloy is notablyhigher than for commercial Al of high purity. This may be explained bythe heat treated alloy having a much lower level of parasitic corrosion.

Example 5 Anode Coulombic Efficiency Test

The measurement of the coulombic efficiency was carried out in a singlecell having technical parameters as follows:

Air electrode size: 7.5×7.3 cmNumber of air electrodes: 2Numbers of anodes: 1Anode thickness: 0.24 cmAnode working area 52.2 cm2 (7.35×7.1 cm)Distance between air electrodes: 0.7 cmTotal volume of circulating electrolyte: 300 ml

The anode material was Al—Mg2.5% alloy after Solid Solution HeatTreatment and 65% deformation by rolling.

The parameters and the test results are as follows:

Discharge current: 10.5 ADischarge current density: 100 mA/cm²Discharge average voltage: 1.3VTime of discharge: 1.1 hTemperature of electrolyte: 41-43° C.Total discharge capacity: 11.5 Ah

The anode weight loss during the test was 4.324 g which corresponds tothe capacity of:

4.32 g×2.98 Ah/g=12.88 Ah.

The coulombic efficiency is calculated as:

[1−(12.88 Ah−11.5 Ah):12.88 Ah]×100=89.3%.

As shown from the test results there is a very good correlation betweenthe coulombic efficiency of the Al—Mg anode in the half cell test (88%in table 5) and in the real Al-Air cell (89.3%—taking into account thedifference in the temperature of electrolyte: 50° C. in a half cell vs.43° C. in Al-Air cell).

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

1. A method for preparing aluminum-based anodes, the method comprising:performing a solid solution heat treatment on an aluminum-based alloy,wherein the aluminum-based alloy comprises at least one alloying elementselected from Mg, Ga, Ge, Ag or Si and the solid solution heat treatmentresults in a metastable supersaturated solid solution phase; andperforming on the aluminum-based alloy a treatment of artificial aging,plastic deformation or a combination thereof to decompose the metastablesupersaturated solid solution phase and produce an aluminum-based anode.2. The method according to claim 1, wherein the metastablesupersaturated solid solution phase is decomposed by plasticdeformation.
 3. The method according to claim 1, wherein the at leastone alloying element is magnesium.
 4. The method according to claim 3,wherein the magnesium is present in an amount of 0.5 to 10% w/w.
 5. Themethod according to claim 3, wherein magnesium is present in an amountof 2 to 4% w/w.
 6. An aluminum-based anode prepared according to amethod comprising: performing a solid solution heat treatment on analuminum-based alloy, wherein the aluminum-based alloy comprises atleast one alloying element selected from Mg, Ga, Ge, Ag or Si and thesolid solution heat treatment results in a metastable supersaturatedsolid solution phase; and performing on the aluminum-based alloy atreatment of artificial aging, plastic deformation or a combinationthereof to decompose the metastable supersaturated solid solution phaseand produce an aluminum-based anode.
 7. The aluminum-based anodeaccording to claim 6, wherein the metastable supersaturated solidsolution phase is decomposed by plastic deformation
 8. Thealuminum-based anode according to claim 6, wherein the at least onealloying element is magnesium.
 9. The aluminum based anode according toclaim 8, wherein the magnesium is present in an amount of 0.5 to 10%w/w.
 10. The aluminum based anode according to claim 8, whereinmagnesium is present in an amount of 2 to 4% w/w.
 11. An aluminum basedanode, comprising Mg as the alloying element, having a coulombicefficiency of at least 85%, at a temperature in the range of 40-50° C.